A METHOD FOR CASCADING OF MICROBASES

Abstract: In a radiotelephone communication system of the type having a plurality of cascaded connected base stations or microbases, a PCM link transmits a TDM signal having a plurality of frames that are divided into time slots that correspond to control time slots and voice channels. At a source base station, the frame synchronization position of the TDM signal is preferably moved to a time slot that is in the intended control time slot for a destination base station. The TDM signal is then transmitted to the destination base station, and the received TDM signal at the destination base station includes a new frame structure having consecutively renumbered time slots. The method of the present invention allows standardized equipment to be used at both the source and destination base stations.

Description: A METHOD FOR CASCADING OF MICROBASES BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for connecting base stations in a radiotelephone communication system, and more particularly, to a method for cascade connecting a plurality of base stations or icrobases in a cellular telephone system. Background of the Invention In order to increase the capacity of cellular telephone systems, it is necessary to reduce the size of cells. Recent proposals have included the introduction of so-called microcells which include a base station and which cover an area of 100-500 meters from the base station. The typical vertical height of an antenna for a microcell base station is approximately the same as street lighting. Microcells are particularly well suited for urban areas which have a large amount of traffic and considerable amounts of interference. In order to further increase capacity, even smaller cells, often referred to as picocells, have been proposed. Picocells are primarily intended for indoor use. The base station of a picocell can cover an area of approximately 20-30 meters. Localized microcells and picocells may be established within overlying macrocells to handle areas with relatively dense concentrations of mobile users, sometimes referred to as "hot spots". Typically, microcells may be established for thoroughfares such as crossroads or streets, and a series of microcells may provide coverage of major traffic arteries such as highways. Microcells may also be assigned to large buildings, airports, and shopping malls. Picocells are similar to microcells, but normally cover an office corridor or a floor of a high- rise building. Microcells allow additional communication channels to be located in the vicinity of actual need, thereby increasing cell capacity while maintaining low levels of interference. The design of future cellular systems will likely incorporate acrocells, indoor microcells, outdoor microcells, public microcells, and restricted microcells. Macrocell umbrella sites typically cover radii in excess of 1 kilometer and serve rapidly moving users, for example people in automobiles. Microcell sites are usually low power, small radio base stations or microcells, which primarily handle slow moving users such as pedestrians. Each microbaεe of a microcell may be connected to a macrocell base station through digital radio transmissions, cables or optical fibers. Alternatively, the microbases of the microcells may be connected directly to a MSC (mobile switching center) via a PCM (pulse code modulated) link that transmits time division multiplexed (TDM) signals having time slots which correspond to control time slots and voice channels for each base station or microbase. When the microbases of microcells are connected to a MSC via a PCM link, they typically are in turn cascade connected to one another. This cascade connection requires a relatively complex switching facility at each node or microbase for modifying the positions of control time slots of the TDM signals transmitted over the PCM link. Accordingly, there is a need for an improved method for cascade connecting microbases which can eliminate the relatively complex switching facilities. SUMMARY OF THE INVENTION The present invention provides a novel method for cascade connecting a plurality of base stations or microbases via a PCM link that does not require a relatively complex switching facility for modifying the position of control time slots of the TDM signals. The base stations of the present invention are of a type that receive over the PCM link a TDM signal having a plurality of frames that are divided into time slots that correspond to control time slots and voice channels. The TDM signal is transmitted to a first or source base station from a MSC. At the source base station, the frame synchronization position of the TDM signal is preferably moved to a time slot that is the intended control time slot of a second or destination base station. The TDM signal is then transmitted to the destination base station, and the received TDM signal at the destination base station includes a new frame structure having consecutively renumbered time slots. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a typical multi-layered cellular system employing umbrella macrocells, microcells and picocells; Fig. 2 is a diagram illustrating two microcell base stations cascade connected to a MSC; Fig. 3 is a diagram of a TDM signal used for communications between a MSC and base stations or microbases of a conventional system; Figs. 4a and 4b are block diagrams of two different embodiments of the present invention having cascaded microbases; and Fig. 5 is a diagram of a TDM signal used for communications between a MSC and base stations or microbases of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is an exemplary multi-layered cellular system. An umbrella macrocell 10 represented by a hexagonal shape makes up an overlying cellular structure. Each umbrella cell may contain an underlying microcell structure. The radio coverage of the umbrella cell and an underlying microcell may overlap or may be substantially non- overlapping. The umbrella cell 10 includes microcells 20 represented by the area enclosed within the dashed line and microcells 30 represented by the area enclosed within the dotted line corresponding to areas along city streets, and microcells 40, 50, and 60, which cover individual floors of a building. The intersection of the two city streets covered by the microcells 30 and 40 may be an area of dense traffic concentration, and thus might represent a hot spot. Briefly, control channels are used for setting up calls, informing the base stations about location and parameters associated with mobile stations, and informing the mobile stations about location and parameters associated with the base stations. The base stations listen for call access requests by mobile stations and the mobile stations in turn listen for paging messages. Once a call access message has been received, it must be determined which cell should be responsible for the call. Generally, this is determined by the signal strength of the mobile station received at the nearby cells. Next, the assigned cell is ordered, by the mobile switching center (MSC) for example, to tune to an available voice channel which is allocated from the set of voice or traffic channels accessible to the assigned cell. Referring now to Fig. 2, a diagram illustrates two microcell base stations BSI and BS2 that are cascaded connected to a mobile switching center MSC via a PCM trunk or link Tl. The PCM link Tl is preferably of a type that operates at either 1.5 or 2.0 Mbits. American systems typically operate at 1.5 Mbits, and European systems typically operate at 2.0 Mbits. The TDM signal transmitted on the PCM link Tl is also of a type which is divided into a plurality of frames and time slots with control time slots and voice channels assigned to predetermined time slots. When more than one base station is connected via the same connection Tl, each base station requires its own time slot for control. Referring now to Fig. 3, a TDM signal of a type that is conventional in the art is illustrated as being divided into a plurality of frames and time slots #1-#18. In the TDM signal transmitted over a conventional PCM link, where for example, base station BSI uses time slot #9 for a control time slot and time slots #l-#8 for voice channels. The time slot #9 cannot be used as the control time slot for the base station BS2. Time slot #10 is used as the control time slot for the base station BS2, and time slots #11-#18 are used as voice channels for base station BS2. Conventional base stations BSI may include a relatively complex switching facility which is used to rearrange the position of time slot #10 to the position of time slot #9. The rearrangement of the time slot #10 to the standard position of time slot #9 is indicated by the arrow A of Fig. 3. The relatively complex switching facility which performs the rearrangement of time slot #10 represents a significant problem in the design of a compact microbase. Referring now to Fig. 4a, a block diagram illustrates a first embodiment of the present invention. In the first embodiment, the PCM link Tl connects a MSC to a source base station BSI which includes a frame synchronization means FS1. The source base station is in turn connected to a destination base station BS2 which includes frame synchronization means FS2. If the destination base station BS2 is in turn connected to a third base station BS3, then the third base station BS3 could include frame synchronization means, but the frame synchronization means does not have to be activated. Referring now to Fig. 4b, a block diagram illustrates a second embodiment of the present invention. In the second embodiment, the PCM link Tl connects a MSC to a source base station BSI. The source base station is in turn connected to a destination base station BS2 which includes frame synchronization means FS2. The destination base station BS2 may be in turn connected to a third base station BS3 that includes frame synchronization means FS3. It should be emphasized that in the preferred embodiments of the present invention, all base stations are equipped with frame synchronization means. According to the first alternative in Fig. 4a, however, the last base station in the cascaded chain has its frame synchronization means deactivated, and according to the embodiment of Fig. 4b, the first base station BSI has its frame synchronization means deactivated. Referring now to Fig. 5, a diagram illustrates a TDM signal transmitted over a PCM link to the microbases of the present invention that avoid the problem associated with the TDM signal of a conventional PCM link. The solution provided by the method of the present invention is to always use the time slot #0 as the control time slot for each cascade connected base station. In the TDM signal of Fig. 5, there are twenty four time slots #0-#23. In the path between the MSC and base station BSI of Figs. 4a and 4b, time slot #0 is used for control of source base station BSI; time slots #l-#6 are used for voice channels of source base station BSI; time slot #7 is used for control of destination base station BS2; and time slots #8-#13 are used for voice channels of destination base station BS2. It should be noted that the voice channels of the present invention are capable of transmitting either voice or data communications. In order to practice the method of the present invention in accordance with the first embodiment of Fig. 4a, the synch position of the TDM signal transmitted over the PCM link is preferably moved at source base station BSI from its original position at time slot #0 and repositioned at time-slot #7. Alternatively in the second embodiment of the invention illustrated in Fig. 4b, the synch position of the TDM signal is moved at the destination base station BS2. According to the alternative second embodiment, the first base station BSI never moves the synch position upon reception of the TDM signal from the MSC. Moving the synch position of the TDM signal from time slot #0 to time slot #7 results in a renumbering of the frames and time slots of the TDM signal at the destination base station BS2. In Fig. 5, this renumbering of the time slots is indicated in the lower portions of time slots #7- 13 which are renumbered as time slots #0-#6, respectively. Accordingly, in the path between the source base station BSI and the destination base station BS2, the renumbered time slot #0 is used for control by the base station BS2, and the renumbered time slots #l-#6 are used for voice channels of the destination base station BS2. This arrangement results in the same TDM signal structure being used for both base stations BSI and BS2, and this arrangement permits standardized equipment to be used at both base stations. It should be noted that the present invention is not limited to the use of two base stations and that additional base stations, such as base station BS3, may be cascade connected in accordance with the teachings of the present invention. Moving the synch position of the TDM signal of the present invention is a relatively easy procedure compared to the movement of time slots as described above in connection with the TDM signal of Fig. 3. The synch position of the TDM signal is moved at either the source base station BSI or the destination base station BS2 by using the frame synchronization means FS1 or FS2. The frame synchronization means FS1, FS2 preferably include a counter to delay the signal or suitable software routines for the base stations BSI or BS2 which introduce an appropriate delay into the TDM signal. Accordingly, moving the synch position of the TDM signal in accordance with the present invention advantageously eliminates the need for the complicated switching facility that is required for rearrangement of the position of a control time slot of the TDM signal of Fig. 3. While the present invention has been described in its preferred embodiments, it is to be understood that the words used are words of description rather than of limitation, and that changes to the purview of the present claims may be made without departing from the true scope of the invention in its broader aspects.

Claims: CLAIMS : 1. A method for communicating on a PCM link between cascade connected base stations of the type that receive signals having a plurality of frames that are divided into time slots, the time slots corresponding to control time slots and voice channels, comprising the steps of: transmitting the signal to a source base station, the transmitted signal including control time slots for at least two cascaded base stations; and moving the frame synchronization position of the transmitted signal from the control time slot of the source base station to a time slot that is the intended control time slot of a destination base station; wherein the signal at the destination base station includes a new frame structure having consecutively renumbered time slots. 2. A method according to claim 1 wherein the moving of the frame synchronization position occurs at the source base station before transmission on the PCM link. 3. A method according to claim 1 wherein the moving of the frame synchronization position occurs at the destination base station upon reception from the PCM link. 4. A method according to claim 1 wherein the base stations are microbases. 5. A method according to claim 1 wherein the PCM link is a 1.5 Mbit link. 6. A method according to claim 4 wherein the PCM link is a 1.5 Mbit link. 7. A method according to claim 1 wherein the PCM link is a 2.0 Mbit link. 8. A method according to claim 4 wherein the PCM link is a 2.0 Mbit link. 9. A method ac ording to claim 1 wherein the time slot #0 is used for control. 10. A method according to claim 2 wherein the time slot #0 is used for control. 11. A method according to claim 3 wherein the time slot #0 is used for control. 12. A method according to claim 4 wherein the time slot #0 is used for control. 13. A method according to claim 5 wherein the time slot #0 is used for control. 14. A method according to claim 6 wherein the time slot #0 is used for control. 15. A method according to claim 7 wherein the time slot #0 is used for control. 16. A method according to claim 8 wherein the time slot #0 is used for control. 17. A plurality of cascade connected base stations of the type that receive signals having a plurality of frames that are divided into time slots, the time slots corresponding to control time slots and voice channels, comprising: means for transmitting the signal to a source base station, the transmitted signal including control time slots for at least two cascaded base stations; and means for moving the frame synchronization position of the transmitted signal from the control time slot of the source base station to a time slot that is the intended control time slot of a destination base station; wherein the signal at the destination base station includes a new frame structure having consecutively renumbered time slots. 18. An apparatus according to claim 17 wherein the means for moving the frame synchronization position is disposed at the source base station in accordance with transmission on the PCM link. 19. An apparatus according to claim 17 wherein the means for moving of the frame synchronization position is disposed at the destination base station in accordance with reception on the PCM link. 20. An apparatus according to claim 17 wherein the base stations are microbases. 21. An apparatus according to claim 17 wherein the PCM link is a 1.5 Mbit link. 22. An apparatus according to claim 20 wherein the PCM link is a 1.5 Mbit link. 23. A apparatus according to claim 17 wherein the PCM link is a 2.0 Mbit link. 24. An apparatus according to claim 20 wherein the PCM link is a 2.0 Mbit link. 25. An apparatus according to claim 17 wherein the time slot #0 is used for control. 26. An apparatus according to claim 18 wherein the time slot #0 is used for control. 27. An apparatus according to claim 19 wherein the time slot #0 is used for control. 28. An apparatus according to claim 20 wherein the time slot #0 is used for control. 29. An apparatus according to claim 21 wherein the time slot #0 is used for control. 30. An apparatus according to claim 22 wherein the time slot #0 is used for control. 31. An apparatus according to claim 23 wherein the time slot #0 is used for control. 32. An apparatus according to claim 24 wherein the time slot #0 is used for control.

Inventors: DJUPHAMMAR HAAKAN OLOV, DJUPHAMMAR, HAAKAN, OLOV
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1994
Main classification: H04J3
IPC codes: H04J3, H04W88